Friction stir processes (FSP) are important for enhancing mechanical properties of metal
sheets, such as the tensile strength, the elongation, etc. The stress distribution of the tool pin is
affected by the thermo-mechanical characteristics of the workpiece in FSP. Recently, magnesium
alloy AZ31 is widely used in machine industries due to the light-weight material property. In this
paper, a thermo-mechanical model for FSP using three dimensional FEM analyses is proposed for
exploring temperature distributions, strain distributions and stress distributions of the workpiece. The
heat generated from the plastic deformation and the friction between the head tool and workpiece is
considered as the heat source in the simulation of the FSP process. A commercial finite element code
– DEFORM 3D is used to carry out the simulation of the plastic deformation of AZ31 sheets during
the FSP. The analytical results of temperature, strain and stress distributions of the workpiece and
head tool can provide useful knowledge for tool pin design in FSP